Tunable integrated RF filter having switched field effect capacitors

ABSTRACT

A filter ( 3 ) is described, which filter is provided with field effect (FET) capacitors (M 1 - 32;  M′ 1 - 32 ) arranged for controlling their respective capacity values, each such FET capacitor (M 1 - 32;  M′ 1 - 32 ) having a source (S) and a drain (D). The source (S) and the drain (D) of each FET capacitor (M 1 - 32;  M′ 1 - 32 ) are coupled to one another. The filter acting as an impedance transformer is a passive low power consuming and tunable filter, such as for a radio frequency (RF) receiver. It occupies only a very small area, while integrated on chip.

[0001] The present invention relates to a filter provided with fieldeffect (FET) capacitors arranged for controlling their respectivecapacity values, each such FET capacitor having a source and a drain.

[0002] The present invention also relates to a receiver and/ortransmitter provided with such a filter.

[0003] Such a filter is known from U.S. Pat. No. 4,092,619. The filterdisclosed therein concerns a metal oxide semiconductor field effect(MOSFET) voltage controlled low-pass filter. The MOSFET filter comprisesa plurality of easy to integrate semiconductor filtering devices coupledin series through an active buffer, which may be a source follower. Thesemiconductor filtering devices each have a control input or gate andform capacitors, whose capacitor values are controlled by a voltagesource connected to the gates. The controllable capacitor values enablethe filter cut-off frequency to be controllable too. This way a tunable,but not solely passive low-pass filter for only the audio range isrealised.

[0004] Therefore it is an object of the present invention to provide atunable, but passive filter, capable of also operating in a higherfrequency range, such as the radio frequency (RF) range and capable ofbeing integrated on chip, on only a limited IC area.

[0005] Thereto the filter according to the invention is characterised inthat the source and the drain of each FET capacitor are coupled to oneanother.

[0006] It is an advantage of the filter according to the presentinvention that it is also capable of operating in the RF range and thatit can in particular be used as a tunable band-pass filter in areceiver, transmitter or combined transceiver. The filter is entirelypassive and therefore does virtually not consume a lot of power.Although the coupled source drain could be used as a control input forcontrolling the capacitor values of the respective FET capacitors, it ispreferred to control the FET capacitors on their respective gates. Thisis because if the gate would be present in the signal path, this wouldresult in a necessary so called Electro Static Discharge (ESD) resistorin said signal path, which would decrease a wanted high Q-factor of thefilter.

[0007] An embodiment of the filter according to the invention ischaracterised in that each FET capacitor has a control input for voltagedependent capacity value control.

[0008] This control input will thus preferably be the gate of the FETcapacitor, also because advantageously the gate current is practicallyvirtually neglectable. A high Q-factor of the filter results, becausethe above mentioned ESD resistor is omitted in the signal path.

[0009] A further embodiment of the filter according to the invention ischaracterised in that the filter is provided with control means coupledto FET capacitor control inputs.

[0010] These control means may even advantageously be simple switchingmeans and/or decoding means capable of switching one or more FETcapacitors or FET capacitor banks into the signal path of the filterand/or capable of correspondingly controlling the capacitor valuesconcerned respectively.

[0011] A still further embodiment of the filter according to theinvention is characterised in that the FET capacitors are split inequally controlled pairs of FET capacitors.

[0012] This is particularly advantageous if the filter is built up as asymmetrically filter having a symmetrical input and a symmetricaloutput, because it provides a large degree of design freedom.

[0013] At wish two or more of the FET capacitors are connected inseries, in order to allow larger capacitor values to be created, whichin addition are variable over a large capacitor value range.

[0014] Preferably the FET capacitors are metal oxide semiconductor(MOSFET) capacitors, capable of providing small but accuratelycontrollable capacitor values, in order to enable the filter accordingto the invention to be an RF filter, which is particularly suitable foroperating in the radio frequency range.

[0015] The present invention also relates to a transmitter, receiver, ortransceiver having an above mentioned filter, which filter is providedwith field effect (FET) capacitors arranged for controlling theirrespective capacity values, each such FET capacitor having a source anda drain, and characterised in that the source and the drain of each FETcapacitor are coupled to one another.

[0016] The above advantages also apply to the transmitters, receiversand/or transmitter/receivers concerned.

[0017] At present the filter according to the invention will beelucidated further together with its additional advantages whilereference is being made to the appended drawing, wherein similarcomponents are being referred to by means of the same referencenumerals. In the drawing:

[0018]FIG. 1 shows a front end for a schematically depicted receiver,which is provided with a filter according to the invention; and

[0019]FIG. 2 shows an embodiment of the filter according to theinvention, which is implemented in an radio frequency (RF) filter.

[0020]FIG. 1 shows in particular an RF front-end 1 of a receiver orreceiver part of for example a transmitter/receiver. The receiver partis generally coupled to mixing means (not shown). The RF front-end 1 asshown comprises an antenna 2, a filter 3 in the form of an RF filter,and a low noise amplifier section 4, respectively coupled to oneanother. The RF filter 3 is mostly equipped as a band-pass filter, inorder to filter out the relevant frequency range, which is thenprocessed further by the low noise amplifier and mixer sections. Recentattempts have been made to fully integrate a receiver on a IC chip areahaving only a minor area. This poses very strong demands on thecomplexity of the circuitry to be integrated, in particular concerningcircuit characteristics, heat developed on chip, power consumption andthe like. Nevertheless a sufficient amount of design flexibility has tobe maintained. This also concerns the filters, such as the RF filter 3,which also has to provide accurate and reproducible filtercharacteristics. In addition the filter 3 has to be tunable and has toprovide for input impedance and output impedance matching andadjustment.

[0021]FIG. 2 shows an embodiment of such a filter 3, which isimplemented as a tunable RF filter. The filter 3 is an RF band-passfilter having a high quality (Q) factor. The filter 3 also transforms anRF input impedance z_(i) at filter input 5 to an RF output impedance Z₀at filter output 6. Basically the filter 3 as shown comprises capacitorsC₁, C₂ and coil L. Additional FET, such as MOSFET capacitors may beadded to the filter 3. In this case filter input 5 is coupled to twoFETs M0 and M′0, whose respective gates G0 and G′0 are coupled toswitching means 7 providing a switching signal V_(switch) for switchingthe filter 3 ON and OFF. In this case N-FETs and P-FETs M0 and M′0 actas filter switches. Apart from possible high frequency blocking coils(not shown) the filter input 5 is coupled to a series arrangement ofpairs of—in this case each 32—FET capacitors M1 . . . M32, in additionto further FET capacitor pairs M′1 . . . M′32, which are also added tothe filter 3 after the capacitors C₁. The filter 3 comprises controlmeans (Contr.), here in the form of separate decoders 8 and 9, coupledto each of the gates G1 . . . G32 and G′1 ... G′32 respectively. Drains(D) and sources (S) of each of the pairs MOSFET capacitors M1 . . . M32and M′1 . . . M′32 are short circuited, as shown. By properlyimplementing the split up decoders 8 and 9 to provide respective controlsignals—in this case voltages—at said respective control gates G1 . . .G32 and G′1. . . G′32 in order to control their respective voltagedependent capacitor values, a proper high Q tuning as well as adjustmentof a wanted input-output impedance matching can be achieved, while thepower consumption due to the filter 3 being passive is minimized.

[0022] Off course the filter 3 may be designed as a symmetrical ornot-symmetrical low-pass filter, a high-pass filter, or a band-stop orband-pass filter or any wanted combination thereof. All types ofsuitable capacitor controllable FET semiconductors may be used as FETcapacitors. Whilst the above has been described with reference toessentially preferred embodiments and best possible modes it will beunderstood that these embodiments are by no means to be construed aslimiting examples of the filter concerned, because variousmodifications, features and combination of features falling within thescope of the appended claims are now within reach of the skilled person.

1. A filter (3) provided with field effect (FET) capacitors (M1-32;M′1-32) arranged for controlling their respective capacity values, eachsuch FET capacitor (M1-32; M′1-32) having a source (S) and a drain (D),characterised in that the source (S) and the drain (D) of each FETcapacitor (M1-32; M′1-32) are coupled to one another.
 2. The filter (3)according to claim 1, characterised in that each FET capacitor (M1-32;M′1-32) has a control input (G1-32; G′1-32) for voltage dependentcapacity value control.
 3. The filter (3) according to claim 1 or 2,characterised in that the filter (3) is provided with control means(Contr.) coupled to FET capacitor control inputs (G1-32; G′1-32).
 4. Thefilter (3) according to one of the claims 1-3, characterised in that theFET capacitors (M1-32; M′1-32) are slit in equally controlled pairs ofFET capacitor s (M1-32; M′1-32).
 5. The filter (3) according to one ofthe claims 1-4, characterised in that the filter (3) is built up as asymmetrically filter (3) having a symmetrical input (5) and asymmetrical output (6).
 6. The filter (3) according to one of the claims1-5, characterised in that two or more of the FET capacitors (M1-32;M′1-32) are connected in series.
 7. The filter (3) according to one ofthe claims 1-6, characterised in that the FET capacitors ()M1-32; M′1-32are metal oxide semiconductor (MOSFET) capacitors (M1-32; M′1-32).
 8. Atransmitter, receiver, or transceiver having a filter (3) according toone of the claims 1-7, which filter (3) is provided with field effect(FET) capacitors (M1-32; M′1-32) arranged for controlling theirrespective capacity values, each such FET capacitor (M132; M′1-32)having a source (S) and a drain (D), characterised in that the source(S) and the drain (D) of each FET capacitor (M1-32; M′1-32) are coupledto one another.